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The Course of the Oxidation of the Aldose Sugars by Bromine Water

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The Course of the Oxidation of the Aldose Sugars by Bromine Water 3 RP418 THE COURSE OF THE OXIDATION OF THE ALDOSE SUGARS BY BROMINE WATER By H. S. Isbell and C. S. Hudson ABSTRACT The optical rotation of a buffered glucose solution upon bromine oxidation rises to a point which corresponds approximately with the rotation of the delta gluconic lactone; then the rotation decreases to a minimum value at a rate corresponding to the hydrolysis of the delta lactone; and finally the rotation slowly increases to a point corresponding to the equilibrium rotation of gluconic acid. These changes agree qualitatively with the hypothesis that the delta gluconic lactone is formed in solution immediately after the oxidation of the sugar by bromine water. Apparently the 1, 5 ring form of glucose is oxidized directly to the 1, 5 gluconic lactone, but the authors believe that further data must be obtained before this mechanism is definitely established. The oxidation of various sugars was followed by the same method and similar changes were observed which indicate that the aldose sugars in general are oxidized by bromine water in slightly acid solution to the delta lactones rather than to the sugar acids as previously believed. CONTENTS Page I. Introduction 327 II. Bromine oxidation of glucose 328 III. Bromine oxidation of various sugars 331 1. Oxidation of d-galactose 332 2. Oxidation of Z-arabinose 333 3. Oxidation of d-xylose 333 4. Oxidation of lactose 334 IV. Experimental details 335 I. INTRODUCTION The oxidation of the aldose sugars to monobasic acids by bromine water, a classical reaction which was originally introduced by H. Kiliani, 1 has been considered as evidence for the aldehydic structure for glucose and related sugars. The mechanism of the reaction has been explained on the hypothesis that in an aqueous solution of glucose an aldehyde tautomer exists in small quantity in equilibrium with the normal form. As this aldehyde form is used up by oxidation the equilibrium disturbance causes a new portion to be formed; 2, *• 5 finally, all the sugar reacts as an aldehyde. ' CH2OH (CHOH) 4 .CHO + Br2 + H20->CH2OH (CHOH) 4 .COOH + 2HBr Aldose Aldonic acid i H. Kiliani, Ann., vol. 205, p. 182, 1880; also Kiliani and Kleeman, Ber., vol. 17, p. 1298, 1884. * Pringsheim, " Zuckerchemie, " Leipzig, p. 8, 1925. 3 Crammer, "Les Sucres," Gaston Doin & Co., Paris, p. 22, 1927. * Armstrong, "The Carbohydrates and the Glucosides," Longmans, Green & Co., London, p. 67, 1924. * Haworth, "Constitution of Sugars," Edward Arnold & Co., London, p. 4, 1929. 327 328 Bureau of Standards Journal of Research [vol. 8 This concept is based upon the fact that when 1 molecule of bromine reacts with 1 molecule of glucose the final products which have been isolated are 2 equivalents of hydrogen bromide and 1 molecule of gluconic acid. The isolation of these products does not determine the mechanism of the reaction because the sugar after oxidation may- pass through a number of steps prior to the separation of the final products. This is particularly true in regard to the sugar acids as they are interconvertible with their lactones. In 1914 Nef 6 showed that gluconic acid forms two lactones rather than one. The second lactone he called a beta lactone, while the one previously known was considered as a gamma lactone; Nef's beta lactone is now believed to be a 1, 5 or delta lactone. The two lactones contain five and six membered oxygen rings in which respect they are analogous to the gamma and the normal forms of glucose. A freshly prepared aqueous solution of gluconic acid on standing forms an equilibrium between the acid and the two lactones. As shown by Levene and Simms 7 the delta lactone is formed rapidly while the gamma lactone is formed more slowly. By utilization of the different rates of formation either lactone may be separated at will. In the past the sugars have been regarded as being oxidized to the acids, the lactones being secondary products. It will be shown in the next paragraph that this funda- mental concept must be altered in order to account for the experi- mental facts which are given. II. BROMINE OXIDATION OF GLUCOSE In a previous paper 8 the authors give an improved method for the preparation of aldonic acids which differs from previous methods in that during the oxidation of the sugar with bromine water a slightly acid reaction is maintained by means of a buffer. The reaction is rapid and nearly quantitative and hence it is possible to follow the course of the reaction by the optical rotation of the solution. When the reaction was followed in that manner a series of peculiar changes in optical rotation was observed. As illustrated in Curve I of Figure 1 the specific rotation of a buffered solution of glucose on oxidation increases in a few minutes to a maximum value, then decreases rapidly to a minimum and thereafter slowly increases and finally becomes constant. When the oxidation is interrupted by removing the free bromine a mutarotation of the solution occurs. If the bromine is removed with sodium thiosulphate the mutarotation which follows is very rapid, as illustrated in Curve III of Figure 1, but if the bromine is removed without altering the acidity of the solution the specific rotation changes more slowly, as in Curve II. This may be accomplished by shaking the solution with olive oil. Since the bromine combines directly with the oil there are no objectionable by-products and no marked changes in acidity. If the bromine is removed with olive oil shortly after the specific rotation of the original oxidation mixture reaches a maximum, the rotation of the resulting solution decreases to a minimum and thereafter slowly rises to a constant value. These changes are entirely different from the changes characteristic of a 6 Nef, Ann., vol. 403. p. 325, 1914. 7 Levene and Simms, J. Biol. Chem., vol. 65, p. 31, 1925. « O. S. Hudson and H. S. Isbell, J. Am. Chem. Soc, vol. 51, p. 2225, 1929; also in B. S. Jour. Research, vol. 3, p. 57, 1929. — : Isbell I Hudson] Oxidation of Aldose Sugars by Bromine Water 329 gluconic acid solution. The specific rotation of a freshly prepared solution of gluconic acid (Curve IV, fig. 2) decreases from a negative value to zero, and finally becomes positive, and thereafter increases very slowly. Inasmuch as the specific rotation of gluconic acid is less than that of any known form of glucose if gluconic acid were formed initially the specific rotation would decrease rather than increase. The initial rise in the optical rotation of the solution of glucose on bromine oxidation shows that gluconic acid is not the primary product of the oxidation of glucose in acid solution. As a consequence, we must seek a new interpretation for the mechanism of this important reaction. With the experimental conditions used in the reaction under discussion a certain ring form of the sugar +T0 460 K *10 V\ j {S+40 «s i ^ [^ -««£ — fe,3o i- ^ to o *» Q--50 1 * o5 <> 5D 10 1! 2 30 Z50 4<» 3 JO 4<50 (C00 4()00 %w weBO ss Figure 1. Bromine oxidation of glucose I, changes in the specific rotation of the sugar solution during oxidation. II, changes in the specific rotation of a portion of the solution after interrupting the oxidation by removing the free bromine with olive oil. III, changes in the specific rotation of a portion of the solution after removing the free bromine with sodium thiosulphate. might be oxidized rather than the hypothetical aldehydic form. If the sugar ring is not broken a gluconic lactone would be formed directly as indicated by the following equation H H H OH H H H H H OH H . HO O.C.C .0 .CO + Br, HO O.C.C .C .CC- + 2HBr H | OH H OH | OH II I OH H 0H| 1 I According to this concept the oxidation of glucose by bromine con- sists in the transfer of each of the two hydrogen atoms associated with the terminal carbon atom in a molecule of glucose to a mole- cule of bromine, giving a net result of 1 molecule of gluconic lactone and 2 molecules of hydrogen bromide. If the normal form of glu- cose (1,5) were oxidized in this manner the delta or 1,5 gluconic lactone would be formed. This would explain the changes in rota- — 330 Bureau of Standards Journal of Research [Vol. 8 tion observed during the bromine oxidation of the sugar. Thus, if the oxidation product is the delta lactone the initial rise in specific rotation is caused by its formation, the subsequent rapid decrease in, rotation being due to its hydrolysis to gluconic acid, the hydrolysis proceeding until a quasi equilibrium is established between the delta lactone and the acid, thus accounting for the minimum value, and simultaneously a second or gamma lactone is formed very slowly. The formation of the second lactone, continuing long after equilib- rium is established between the delta lactone and the acid, adequately explains the slow rise in rotation. k A comparison of the changes in optical rotation of^glucose on bromine oxidation, of gluconic acid, and the delta lactone, is given in Figure 2, which shows graphically the close agreement in rotation +70 60 < >n t50 P+40 o +.JO |t -luO \ u "«0 o- 1 ^"^ 11 *••• f& t < < lift - O 200 400 600 80O 1000 1200 1400 1600 l&OO £000 2200 Figure 2. Curves showing a comparison in the specific rotation of the oxidation product from glucose with gluconic acid and its delta lactone I, a buffered glucose solution upon bromine oxidation (Table 1).
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